The present invention relates to an exhaust gas purifying method and an exhaust gas purifying system for purifying NOx by a NOx occluding reduction type catalyst and purifying PM by a DPF.
Legal restriction on discharge quantities of NOx (nitrogen oxide) and particulate matter (hereafter referred to as PM) is enforced year by year together with legal restriction on discharge quantities of CO (carbon monoxide) and HC (carbon hydride). Thus, only improvement of an engine cannot manage a restriction value for the enforcement of the restriction. Therefore, a technique is adopted which reduces these matters discharged from an engine by mounting an exhaust gas control system.
Moreover, many NOx purifying catalysts are developed for NOx and a filter referred to as a diesel particulate filter (hereafter referred to as DPF) is developed for the PM.
A NOx occluding reduction type catalyst is one of the NOx purifying catalysts. In the NOx occluding reduction type catalyst, a catalyst metal having an oxidizing function for NOx and a NOx occluding material having a NOx occluding function are supported on a porous catalyst coat layer such as alumina (Al2O3). The catalyst metal is formed by platinum (Pt) and so on. The NOx occluding material is formed by one of or a combination of some of alkaline metals such as sodium (Na), potassium (K), and cesium (Cs), alkaline earth metals such as calcium (Ca) and barium (Ba), and rare earths such as yttrium (Y) and lanthanum (La). The NOx occluding reduction type catalyst shows two functions depending on the O2 (oxygen) concentration in exhaust gas. One is a function of occlusion of NOx. And the other is a function of release and purification of NOx.
First, in the case of an exhaust gas condition (lean air-fuel ratio state) having a high O2 concentration in the exhaust gas such as a normal operational state of a diesel engine or a lean-burn gasoline engine or the like, NO (nitrogen monoxide) is oxidized by O2 contained in exhaust gas as a result of the oxidizing function of the catalyst metal to become NO2 (nitrogen dioxide). The NO2 is occluded in the NOx occluding material in the form of chloride. In this manner, the exhaust gas is thus purified.
However, when occlusion of the NOx continues, the NOx occluding material such as barium is changed to nitrate. Accordingly, the NOx occluding material is gradually saturated to lose the function for occluding NOx. To avoid such situation, over-rich combustion is performed by changing operation conditions of the engine to generate exhaust gas (rich spike gas) having a low O2 concentration, high CO concentration, and high exhaust gas temperature and supply the exhaust gas to the catalyst.
In the rich air-fuel ratio state of the exhaust gas, the NOx occluding material changed to nitrate by occluding NO2 releases the occluded NO2 and returns to the original substance such as barium. Because O2 is not present in the exhaust gas, the released NO2 is reduced on the catalyst metal by using CO, HC, and H2 in the exhaust gas as reducers. That is, these components are converted into N2, H2, O, and CO2. In this manner, the NOx in the exhaust gas is purified.
However, when using the NOx occluding reduction type catalyst, it is impossible to burn a soot component in PM by the catalyst alone. Therefore, as disclosed in Japanese Patent Laid-Open No. 1997-53442, it is required to combine the catalyst with a DPF or integrate the NOx purifying function of the NOx occluding reduction type catalyst with the PM purifying function of the DPF. Moreover, it is required to combine both in order to purify the NOx generated in regeneration of the DPF.
The NOx occluding reduction type catalyst has a problem in that sulfur in fuel is accumulated in the NOx occluding material, and the NOx purifying efficiency is deteriorated as the operation of the engine continues. Therefore, as disclosed in Japanese Patent Laid-Open No. 2000-192811, in spite of difference between the types of the catalyst to be used, it is required to perform sulfur purge control (sulfur desulfurization control) by keeping the exhaust gas flowing into the catalyst in the condition of a temperature higher than approximately 600 to 650° C. and a rich atmosphere.
The sulfur purge control accelerates sulfur purge by bringing the exhaust gas into the rich state and raising the temperature of the catalyst by the oxidation activation reaction heat generated at the catalyst. In the case of a diesel engine, the rich state is realized by reducing the intake volume through intake-air throttling or through a large quantity of EGR and by performing post injection as well as directly adding light oil to a post injection or an exhaust pipe.
However, the sulfur purge for recovering the NOx occluding function of the catalyst by increasing the quantity of sulfur purge has the following problems.
Because the oxygen concentration in exhaust gas is very low under a rich air-fuel-ratio state, the time required to raise the temperature of the catalyst up to a temperature at which the sulfur purge can be made becomes very long. Therefore, fuel consumption is deteriorated. Moreover, the quantity of sulfur purge increases as rich is denser. However, when performing a dense rich state operation, fuel consumption is extremely deteriorated. Moreover, a problem of slip of HC or CO occurs that HC or CO is generated in a large quantity and some of HC or CO is discharge into atmosphere.
Furthermore, in the case of a DPF, a continuously regenerating type DPF is developed which is constituted by combining an oxidation catalyst or the like with the DPF in order to burn and remove PM. In the DPF, the PM can be burned and removed at a comparatively low temperature. However, in a state where an exhaust gas temperature is low and clogging of the DPF progresses, exhaust gas temperature raising control such as an intake-air throttling is performed to temporarily raise the temperature of exhaust gas in order to burn and remove the collected PM.